U.S. Pat. No. 3,758,533 relates to a method for removing polyethylene impurities from tallow and like fatty materials. In the method, the fatty material is heated to a temperature to about 95° C. or below but above the melting temperature of the fatty material to render the polymeric impurities insoluble in the fatty material and separating the impurities. The separation of impurities can be conducted by filtration or centrifuging. According to the teachings of U.S. Pat. No. 3,758,533 separation of polyethylene impurities by settling is not possible since the density of polyethylene is only a little greater than that of tallow (866 kg/m3).
U.S. Pat. No. 7,262,311 relates to a method for removing phospholipids and other impurities from crude vegetable oil using physical refining techniques. In the method, the crude oil is allowed to settle over a period of time, such as 20 days, such that the oil separates into a gums layer and oil layer which are separated from each other. The purified oil contains from 25 ppm to 110 ppm of phosphorus.
U.S. Pat. No. 2,601,375 relates to a method for recovering of vegetable oils from sludge or settlings, which accumulate in vegetable oil storage tanks. In the method, the sludge is mixed and heated with water and the mixture is allowed to cool and settle into an upper oil phase and lower aqueous phase with solid impurities. The separated lower phase can be treated with alkali followed by acid treatment.